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What are Hydrophobic Materials?

Many of us have experienced the blind panic of dropping a mobile phone into water and finding it completely ruined after fishing it out. Even more of us have experienced the daily drudgery of washing dirty clothes. However both these experiences may become a thing of the past thanks to advancements in hydrophobic materials and coatings.

Hydrophobic materials are formed based on the chemical concept of hydrophobicity. Hydrophobicity is a term derived from a Greek term ‘hydro’ meaning water and ‘phobos’ meaning fear. Hydrophobic molecules (or hydrophobes) repel bodies of water and, owing to the fact that hydrophobes are non-polar, they attract other neutral molecules and non-polar solvents. Natural hydrophobes include alkanes, fats and oils.

Hydrophobic materials are often used to remove oil from water, manage oil spills, and chemical separation processes that require the removal of non-polar substances from polar compounds.

Material Properties of Hydrophobic Materials

A material’s surface can react either in a hydrophilic (water-loving) or a hydrophobic (water-hating) manner.

To understand the behavior of a surface towards water, its contact angle (CA) has to be measured, which will provide information on the interaction energy between the surface and the liquid. The angle at which a liquid/vapor interface converges with a solid surface is known as the contact angle. This can be measured using a contact angle goniometer.

The greater the contact angle, the more hydrophobic a surface is. For example, the contact angle of water on lotus is >140, hence that surface is termed superhydrophobic while water on plastic has a contact angle of 90-140, hence that surface is termed hydrophobic.

Material scientists have been working on using various chemicals to alter the surface properties of a surfaces to make them hydrophobic. To provide glass with a hydrophobic characteristic, silanes are often used. The leaves of lotus have been a great source of inspiration in the formation of superhydrophobic materials. Studies revealed that the rough surface of the leaves contain wax nanocrystals, which aids the repulsion of water. The water drops come in contact with a large fraction of air that forces it to form a bead shape and slide off the surface. Another extremely useful benefit of this is that as the water slides off, it also drags the surface dirt with it, rendering the surface clean and water-free.

Similar hydrophobicity concepts have been applied in paints, coatings and textiles.

Berghaus, a U.K.-based outdoor clothing company, have used hydrophobic materials to enhance their clothing range. They coated down with durable water repellent (DWR) before fitting it into Berghaus' clothing. The DWR-coated down retained up to 80% of its loft after 3 min in water, meaning the jacket will not only be water-resistant but will also keep you warm.

How to Make a Hydrophobic Material

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Hydrophobic materials can be created using two methods. The simpler method is to coat a surface with wax, oil, or grease. The other is using nanoengineering to help create a unique, nanopatterned textured surface. The nanopatterns consist of small bumps that have a width of 10 µm.

Researchers at MIT have taken this to another level by coating a nanopatterned hydrophobic material with a thin layer of lubricant, thus greatly enhancing the hydrophobicity of the surface. On a closer look, it was observed that the gaps between the bumps could exert a precise amount of capillary force to hold the lubricant in place. Now the researchers want to test this newly enhanced hydrophobic material in power station cooling towers and desalination plants.

New Innovations

The new hydrophobic material or superhydrophobic material that was created by a team of MIT nanomaterial scientists and mechanical engineers is said to be 10,000 times more hydrophobic than current hydrophobic surfaces. MIT’s superhydrophobic materials are set to revolutionize the efficiency of fossil fuel power plants. Just as this material is very useful on car windshields, it is also very valuable for use in cooling towers.

Fossil fuel and nuclear power plants use steam turbines to produce electricity, and cooling towers to condense the steam back into water. As the inner surface of the cooling tower is not hydrophobic, the water remains within, thus greatly minimizing the tower’s efficiency.

Another revolutionary product that uses the hydrophobic material is a sponge called ‘Obsorb’, which was developed by Paul Edmiston of the College of Wooster. This sponge is designed to absorb oils and solvents in water and grow to about eight times its weight. It is made up of nano-matrix of glass pieces, which absorb volatile organic compound in water without reacting with the water. When the sponge reaches its limit, it floats to the surface. It can then be washed and reused numerous times.

The stand-out properties of Obsorb make it perfect for low-tech, inexpensive cleanups. Three versions of the super sponge are currently being worked upon for companies and government agencies involved in clean-up or remediation of toxic groundwater contamination sites.

The mess left behind by Hurricane Sandy saw raw sewage, petroleum, and industrial chemicals mixed in the receding rainwater making a dangerous sludge. Cleaning this up will take a huge effort and Obsorb could be an ingenious solution. The Obsorb team is also interested in its use for cleaning the large amounts of water that is extracted during oil drilling or fracking. Cleaning up this water would eliminate some of the environmental hazards associated with this controversial drilling practise.

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